Catalysts capable of effecting the copolymerization of epoxides and carbon dioxide to form aliphatic polycarbonates (APCs) have been known in the art since the 1960s. The early catalysts were based on heterogeneous zinc compounds and suffered from low reactivity, a lack of selectivity for polymer formation vs. cyclic carbonate formation, and a tendency to produce polycarbonates contaminated with ether linkages.
Improved catalysts based on transition metals have been discovered over the past decade or so. These newer catalysts have increased reactivity and improved selectivity. Nevertheless, even using highly active catalysts such as those disclosed in U.S. Pat. No. 7,304,172, the reaction times required to make high molecular weight polymer are typically quite long. In addition, the best-performing catalysts disclosed in the '172 patent require the addition of a separate co-catalyst to achieve optimum activity.
Attempts to improve these transition metal catalysts have been made. Catalysts described by Lee and co-workers (WO2008/136591 and Sujith et al., Angew Chem. Int'l. Ed., 120, 7416-7419, 2008) use the approach of tethering co-catalyst moieties directly to the metal ligand and in some cases result in high catalysis rates.
These next-generation catalytic systems suffer from lengthy and complicated syntheses and are therefore expensive. Attempts have been made to recycle these catalysts and their precursors to lower costs over time, but there remains a need for less expensive catalysts with high activity for epoxide CO2 copolymerization.